In general, printers include at least one printhead or ejector head that ejects drops of liquid ink in two dimensional printers and drops of material in three-dimensional object printing onto a surface. In some cases, monitoring of the volume or the head height of the ink or materials stored for ejection is important. Accurate monitoring of the head height is especially important where the head height of a stored fluid affects the mechanism or system that draws or uses the fluid. For example, restricting the head height range within an ink reservoir and precisely controlling the replenishment to an on-board ink reservoir of a printhead are often needed to prevent overfill-caused dripping of ink from the printhead jet orifices and to prevent the introduction of air if the fluid level is depleted below tolerable levels. Air can cause ink to foam and render a printhead inoperative.
Currently available fluid sensing systems suffer from a number of drawbacks. For instance, applications in which small reservoirs or holding tanks are needed to store a fluid may not offer the space or fluid height required to accommodate known fluid sensing systems, such as float-based systems. Also, many “sense and fill” systems suffer from significant hysteresis problems in that these systems tend to respond late or overfill before flow is stopped. Moreover, fluid sensing systems that sense fluid materials by detecting a resistance change upon attaining a liquid level are dependent on consistent material properties, which may change over the life of the mechanism or system that uses the fluid. For example, the properties of a fluid may deteriorate over time due to age degradation, or the fluid may be replaced with a fluid having different properties. This problem is more frequently encountered in three-dimensional object printing because these printers typically store a wider range of materials than inkjet printers. Therefore, improvements to sensing systems that enable fluid sensing in small reservoirs and that can detect fluids with varying properties are desired.